some non-trivial changes to platform.h and util.h,
initially related to compilation for Haiku,
but I used this opportunity to make them cleaner
and add some documentation.
Noticed several tests that could be improved
(too harsh conditions, useless exception, etc.)
but I did not dare modifying too many tests just before release.
by invoking time() once per batch, instead of once per compression / decompression.
Batch is dynamically resized so that each round lasts approximately 1 second.
Also : increases time accuracy to nanosecond
The new macro might be a bit too restrictive.
Systems which do not support new test will simply default to <time.h>'s `clock_t clock()`,
suffering lesser benchmark accuracy.
Should it matter, the detection macro will have to be upgraded.
Currently, all files are joined by default,
they are compressed separately but benchmarked together,
providing a single final result.
Benchmarking files separately make it possible to accurately measure difference for each file.
This is expected to be useful while tuning optimal parser.
UTIL_getFileSize() used to return zero on failure.
This made it impossible to distinguish a failure from a genuine empty file.
Both cases where coalesced.
Adding UTIL_FILESIZE_UNKNOWN constant has many consequences on user code,
since in many places, the `0` was assumed to mean "error".
This is no longer the case, and the error code must be actively checked.
for easier invocation.
- no longer expose frequency timer :
it's either useless, or stored internally in a static variable (init is only necessary once).
- UTIL_getTime() provides result by function return.
The timer used was only accurate up to 0.01 seconds. This timer is accurate up to 1 ns.
It is a monotonic timer that measures the real time difference, not on CPU time.
